Ptychagnostus is a member of the agnostida that lived during the Cambrian period. Ptychagnostidae generally do not exceed one centimetre in length.[1] Their remains are rarely found in empty tubes of the polychaete worm Selkirkia.[2] The genus probably ranged throughout the water column. It has two glabellar lobes, and three pygidial lobes,.[3]
Agnostus punctuosus Angelin, 1851 from the Pt. punctuosus Zone of the Alum Shale (Drumian), Sweden (by original designation). Official ruling on the conservation of accepted usage of A. punctuosus as the type species was given by the International Commission on Zoological Nomenclature, 1993.
Ptychagnostus affinis (Brøgger 1878) [4] was once considered a subspecies of Pt. punctuosus. Laurie (2008) [5] grouped punctuosus and affinis within Ptychagnostus, but preferred to place the closely related atavus within Acidusus.
Høyberget & Bruton (2008) [6] concluded that the following genera belong in the Ptychagnostidae: Ptychagnostus, Goniagnostus, Tomagnostus, Lejopyge, Aotagnostus and Onymagnostus.